When fastening articles together by means of screws or bolts, lock nuts are frequently employed to prevent the fastened articles from becoming detached.
The British patent to Ekstrom et al, U.S. Pat. No. 255,308 shows a lock nut having a projecting collar at one end with an internally threaded bore running the full length of the main body and the collar. After the threads are formed, all of the same initial pitch, Ekstrom proposes to compress the collar axially "only to a certain extent, defined by practice", so as to reduce the thread pitch on that portion of the threads within the collar. He has no teaching as to how that "certain extent" may be determined, other than to say that "the clamping action must neither be too strong nor too weak". Neither does he teach any method or means by which the compression may be controlled. Ekstrom does not teach any necessary limitation of the radial thickness of the collar as compared to the depth of the threads. Neither does he teach any necessary limitation of the relationship between the precompression axial length of the collar and the pitch of the threads. Still further, he does not teach any necessary limitation as to the amount of compression of the collar as compared to the initial thread pitch. All of these limitations are critical and failure to observe then accurately results in nuts having undesirable torque characteristics.
Ekstrom et al shows in FIG. 3 a nut having a constant radius collar 2 before axial compression. FIG. 4 shows the same collar, tapered by axial compression, with a thicker wall adjacent the body 1 and a thinner wall at the end remote from the body. The Ekstrom et al disclosure is misleading, in that it is not possible, solely by axial compression of the collar of FIG. 3 to produce the collar shown in FIG. 4. On the contrary, the collar structure shown in FIG. 4 can be produced from the collar of FIG. 3 only by laterally restraining the upper end of the collar from expansion while it is being compressed axially. Die structures for producing such axial compression and lateral restraint are shown in Henderson British Pat. No. 11,281 of 1885 and Robert French Pat. No. 1,550,065.
The French patent of Ekstrom et al, No. 607,984, which corresponds in part to British Pat. No. 255,308, regards the axial compression of threads and the radial compression thereof as mechanical equivalents, as far as their locking function is concerned.
Stanford, U.S. Pat. No. 1,367,168, shows a reduction of pitch of certain threads of a lock nut by deformation under heat.
Many other prevailing torque lock nuts of the prior art have their threads distorted radially and in some cases both axially and radially. Such lock nuts rely on high pressure radial interference contacts between threads. In use, such contact areas may gall the threads on even tear off parts of the threads, so that the holding torque may be lost or possibly increased to the point that the parts may be separated only with difficulty, if at all.